This invention relates to a process for the treatment of collagenous tissue to render it suitable for use in prosthetic implants, and to the resulting tissue so treated. More particularly, the invention is concerned with a process for the treatment of collagenous tissue to adapt it to be used in a prosthetic implant and to promote the growth of endothelial cells thereon.
Prosthetic implants for use in humans have been known for some time and it also has been known to use natural tissue taken from animals including humans. When natural tissue is used in an implant it is necessary to treat it to avoid problems after implantation, for example excessive mineralization or calcification and rejection by the body's immune system. Numerous treatments for improving the stability of prosthetic devices made from natural tissue have been proposed in the prior art.
Thus U.S. Pat. No. 4,378,224 issued Mar. 29, 1983 to Nimni et al discloses a process for improving the biophysical stability of bioprostheses for heterograft or allograft implantation made from animal tissue involving the formation of cross-links in the protein structure of the tissue using the known cross-linking agent glutaraldehyde and soaking the tissue in an aqueous solution of a calcification inhibitor. Examples of suitable calcification inhibitors mentioned by Nimmi et al are diphosphonates and 3-amino-1-hydroxypropane 1,1-diphosphonic acid is mentioned as a typical diphosphonate, although no specific Example illustrating the use of this compound is given by Nimni et al.
Furthermore, although Nimni et al refer to harvesting and cleaning of the tissue prior to the glutaraldehyde treatment, there is no suggestion of any pre-treatment with an appropriate surfactant to remove, substantially completely, deleterious material present in the tissue. Accordingly, the coating (column 2 line 23) provided by Nimni et al is essentially a surface phenomenon and cross-linking with glutaraldehyde and stabilization with the calcification inhibitor throughout the fibrous matrix of the tissue is not and can not be achieved by the Nimni procedure.
U.S. Pat. No. 3,988,782 issued Nov. 2, 1976 to Dardik et al discloses the preparation of prostheses in the form of tubes, patches and conduits from arteries and veins of umbilical cords using glutaraldehyde as a hardening agent.
U.S. Pat. No. 3,966,401 issued June 29, 1976 to Hancock et al discloses the preparation of an implantable heart valve from porcine pericardial tissue in which the tissue is treated with glutaraldehyde as a tanning agent.
The inhibitory effect of various diphosphonates on aortic and kidney calcification in vivo is discussed in an article by M. Potokar and M. Schmidt-Dunker appearing in Atherosclerosis, 30 (1978) 313-320.
U.S. Pat. No. 4,120,649 issued Oct. 17, 1978 to Schechter discloses the treatment of transplants with glutaraldehyde to enhance the retention time in the recipient. As in Nimni et al, supra, the treatment is essentially a surface treatment and no additional stabilization or like treatment is disclosed.
U.S. Pat. No. 3,562,820 issued Feb. 21, 1971 to Braun, discloses the hardening with glutaraldehyde of tubular, strip and sheet form prostheses based on biological tissue.
U.S. Pat. No. 4,098,571 issued July 4, 1978 to Miyata et al discloses a process for preparing a heterograft substitute blood vessel which comprises treating a pig blood vessel with a proteolytic emzyme to digest unwanted material and retain collagenous and elastic fiber constituents and then fixing the resulting blood vessel with, inter alia, a mixture of formaldehyde and glutaraldehyde.
U.S. Pat. No. 4,323,358 issued Apr. 6, 1982 to Lentz et al discloses treatment of a glutaraldehyde-fixed animal tissue with a solution of a water-soluble salt of a sulfated higher aliphatic alcohol, such as sodium dodecyl sulfate, allegedly to inhibit calcification of the tissue after implantation.
Although all of the above prior art proposals have some degree of success, for example by inhibiting calcification to some extent and improving the biophysical stability of prosthetic implants to some extent, problems in these areas still remain. Furthermore, none of the aforesaid prior art disclosures express any recognition of the importance of promoting and enhancing the growth of endothelial cells on the surfaces of prosthetic implants.
The endothelium is a layer of flat cells lining various cavities within the body, in particular blood vessels. The lining of endothelial cells provides a smooth surface so that blood cells and platelets can flow without being damaged. Endothelial cells are capable of producing and secreting substances with a variety of actions and the actions occuring at the blood-endothelial interface contribute towards the well-being of the organism as a whole; for example, the intact endothelium is nonthrombogenic because both circulating blood cells and the endothelial surface have a negative charge and thus repel each other. Each endothelial cell is closely linked to its adjacent cells and the endothelial layer forms a selectively permeable membrane which resists the passive transfer of the fluid and cellular phases of blood.
While the intact endothelium acts as a primary barrier against the leakage of blood it also provides a prima facie indication to the body's immune system that foreign materials are not present, at least outside the blood vessels. However, if the endothelium is damaged, punctured or broken this automatically induces a response by the immune system which defends against foreign pathogens. The immune system, which is generally capable of discriminating between self and foreign antigens, operates through a complex assortment of lymphocytes and phagocytic cells whose activities are adapted to produce a coordinated protective response to foreign pathogens. Thus, among the phagocytic cells involved in the immune system, white blood cells or leukocytes function primarily to defend the body against microorganisms. Another important group of phagocytes is the macrophages which are widely distributed throughout the body and act in concert with other phagocytes associated with the linings of blood vessels, i.e. the endothelium, in, for example, the bone marrow, liver, spleen and lymph nodes.
A more detailed description of the immune system is not considered necessary for a full understanding of the present invention, but recognition of the role played by endothelial cells is important for an appreciation of the improvement provided by the invention over the prior art.
Surprisingly, it has now been found that by performing the process of the present invention and, in particular, ensuring substantially complete removal of deleterious material from collagenous material by the essential initial step of said process, the in vivo growth of endothelial cells upon prosthetic implants formed from tissue treated by the invention process is promoted. In addition to a marked improvement in the inhibition of mineralization or calcification upon implantation, this permits the formation of implants which are less susceptible to rejection by the body's immune system than any produced by prior art procedures.